This invention relates to a single-ended down converter comprising a transformer, a pulse width controlled switching element in the input circuit and a rectifier diode as well as a fly-wheel diode in the output circuit.
The rectifier diode has a conductive state and a cut-off state caused by switching of the switching element.
In a down converter, energy is transmitted to the load circuit at the output end during the conducting phase of the switching element controlled in pulse width. The magnetization energy taken up by the transformer during the conducting phase of the switching element is still stored in the transformer each time the switching element opens the current circuit at the input end, which is the reason why the transformer must periodically be demagnetized. This can be effected by means of a so-called demagnetization winding additionally provided on the transformer.
It is already known from DE OS No. 2624800 to shunt, in an arrangement without such an additional demagnetization winding on the transformer, the switching element in the input circuit by a series-combination of a diode and a capacitor, a resistor being further connected parallel to the diode. When the switching element is opened, the series-combination of the diode and the capacitor takes over the decaying current of the primary winding. The capacitor is first charged approximately to the value of the input voltage when the influence of the parasitic inductance is neglected. The transformer voltage breaks down and increases again during the demagnetization with inverse polarity. The remagnetization of the transverse inductance of the transformer through the capacitor produces, during the cut-off time, a half period sinusoidal oscillation, which is added at the switching element to the input voltage as a cut-off voltage. The next time that the switching element is switched on, the charge on the capacitor is neutralized through the resistor and the switching element.
In down converters there is a tendency to use increasingly higher switching frequencies because consequently higher power can be transmitted with the same construction volume. Therefore, the circuit arrangements utilized must also be suitable for a high frequency range of, for example, 150 kHz. In this frequency range, the time constant of the switching means provided in the known circuit arrangement for the demagnetization switching element has a very disturbing effect because it no longer allows a sufficiently rapid demagnetization of the transformer.
SUMMARY OF THE INVENTION
An object of the invention is to provide a single-ended down converter of the kind mentioned in the opening paragraph, which is further simplified with respect to the known circuit arrangement and is also especially suitable for higher switching frequencies, for example in the range of 150 kHz and higher.
According to the invention this object is achieved in that the rectifier diode has a barrier layer capacitance present during the cut-off state of the rectifier diode whereby demagnetization of the transformer takes place only by means of said barrier layer capacitance during the cut-off state of this diode. Demagnetization switching means at the switching element at the input end are not required.